Various antigenic moeities exposed for some hours in vitro to macrophages activated by mycobacterial have been found on injection into animals to induce preferentially a cellular immune response. This has been seen also with secondary antigens injected into the site of an ongoing delayed hypersensitive reaction to a primary independent antigen to which the animal had been previously sensitized. In the latter case the observed result is presumed to depend on the presence in the reaction site of immunologically activated macrophages. Observations of these kinds have been made with sheep erythrocytes, with mouse and guinea pig tumors, and with pneumococcal polysaccharide. These results coincide with our thesis that antigens with few determinants are preferential inducers of cellular immunity, as suggested by work of others and ourselves with naturally occurring substances. Consequently, we believe that the role of adjuvant activated or of immunologically activated macrophages in encouraging such responses may lie in their degradation of complex antigens to oligodeterminant fragments. We intend to continue efforts to define this fragmentation by isolating the products of antigenic degradation from activated macrophages and testing them for immunogenic activity, with special regard to induction of cellular immunity. In addition, we wish to know what activation of macrophages consists of, in respect to possible changes of the cyclic nucleotides in such cells following treatment with adjuvants, or after immunologic activation. BIBLOGRAPHIC REFERENCES: Complement-Dependent Anaphylactic Reactions. Infection and Immunity, 11: 1284-1290, 1975 (with B. H. Tom). Reprints submitted 7/17/75.